The locus for properdin (properdin factor complement, Pfc), a plasma glycoprotein, has been mapped to band A3 of the mouse X chromosome by in situ hybridization to metaphase spreads containing an X;2 Robertsonian translocation. The X-linkage of the locus has also been confirmed by analysis of Mus musculus x Mus spretus interspecific crosses. The XA3 localization for Pfc places it in the chromosomal segment conserved between man and mouse which is known to contain at least six other homologous loci (Cybb, Otc, Syn-1 Maoa, Araf, Timp).